1. Field of the Invention
The present invention relates generally to controlling chemical mechanical polishing operations to provide uniform polishing of wafers, and more particularly to controlling a rate of roughening of a polishing surface in relation to a rate at which the polishing surface becomes smoother during chemical mechanical polishing operations performed on a series of wafers.
2. Description of the Related Art
In the fabrication of semiconductor devices, planarization operations are often performed on a semiconductor wafer (“wafer”) to provide polishing, buffing, and cleaning effects. Typically, the wafer includes integrated circuit devices in the form of multi-level structures defined on a silicon substrate. At a substrate level, transistor devices with diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define a desired integrated circuit device. Patterned conductive layers are insulated from other conductive layers by a dielectric material. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to increased variations in a surface topography of the wafer. In other applications, metallization line patterns are formed into the dielectric material, and then metal planarization operations are performed to remove excess metallization.
A chemical mechanical polishing (CMP) process is one method for performing wafer planarization. In general, the CMP process involves holding and contacting a rotating wafer against a polishing surface of a moving polishing pad under a controlled pressure. CMP apparatus typically configure the polishing pad on a rotary table or a linear belt to present the polishing surface to the wafer.
An exemplary CMP apparatus in accordance with the prior art may include a linear polishing pad configured to advance around spaced rollers. A platen is disposed opposite to the polishing surface of the polishing pad to support the polishing pad during a CMP operation. A wafer carrier is configured to hold and urge the wafer against the polishing surface of the polishing pad during the CMP operation. The wafer carrier is driven by a spindle that rotates the wafer while simultaneously urging the wafer against the polishing surface with an appropriate force. Slurry is introduced onto and distributed over the polishing surface of the polishing pad to facilitate and enhance the CMP operation. Additionally, a conditioner is used to condition the polishing surface of the polishing pad during the CMP operation.
An end point of a CMP operation performed on one particular wafer may be defined as the completion of removal of the excess metallization, such as copper, during a metal (e.g., copper) planarization operation performed on that one wafer. In the past, when one such CMP operation has been performed on such one wafer, such end point has been detected by measuring a current drawn by a motor that drives a transmission or other mechanical device connected to one of the spaced rollers around which the linear polishing pad extends. In this CMP operation on the one wafer with the exemplary copper metallization, a change in the current indicated the completion of removal of the copper from that one wafer. However, in such CMP apparatus having this type of end point detection, the current thus only indicated a change in the structure of the one wafer. Thus, apparatus for this end point detection was not configured to provide an indication of other aspects of the CMP operation, such as any effects of the CMP process on the CMP apparatus itself. As a result, motor current has not been relied upon commercially as an indicator of non-end point aspects of CMP operation.
In view of the foregoing, there is a need for improvements in CMP apparatus and methods in which motor current, and related work performed by such a motor during CMP operations, are reliable indicators of aspects of the CMP operations other than the structure of the one particular wafer that is being CMP processed.